Update prometheus-client package to 0.5.0

The update involved some refactoring on their end that eliminated
_LabelWrapper, so this required a minor change (but I don't know why I
was doing it that way in the first place).

tildes/requirements.txt

@@ -44,7 +44,7 @@ Pillow==5.3.0
 plaster==1.0
 plaster-pastedeploy==0.6
 pluggy==0.8.0
-prometheus-client==0.4.2
+prometheus-client==0.5.0
 prompt-toolkit==2.0.7
 prospector==1.1.6.2
 psycopg2==2.7.6.1

tildes/tildes/metrics.py

@@ -10,7 +10,6 @@
 from typing import Callable
 
 from prometheus_client import Counter, Histogram
-from prometheus_client.core import _LabelWrapper
 
 
 _COUNTERS = {
@@ -50,7 +49,7 @@ def incr_counter(name: str, amount: int = 1, **labels: str) -> None:
     except KeyError:
         raise ValueError("Invalid counter name")
 
-    if isinstance(counter, _LabelWrapper):
+    if labels:
         counter = counter.labels(**labels)
 
     counter.inc(amount)
@@ -63,7 +62,7 @@ def get_histogram(name: str, **labels: str) -> Histogram:
     except KeyError:
         raise ValueError("Invalid histogram name")
 
-    if isinstance(hist, _LabelWrapper):
+    if labels:
         hist = hist.labels(**labels)
 
     return hist